Lubricating compositions



United States Patent rnnnrcarnso comosrrrons No Drawing. Application September 14, 1954 Serial No. 456,063

Claims priority, application Great Britain September 17, 1953 3 Claims. (Cl. 252-484) The present invention comprises improvements in or relating to lubricating compositions and relates particularly to fluid lubricants capable of use over a very wide range of temperatures.

The commonly used lubricating compositions based on mineral hydrocarbon oils have certain well-known disadvantages which limit their utility.

Thus, for example a mineral oil which will satisfactorily lubricate an internal combustion engine must have a sufficiently high viscosity to provide adequate lubrication at elevated temperatures and must also afford adequate resistance to Wear of moving parts.

Furthermore it must not be unduly volatile and in order to ensure this, it is found necessary to employ oils of a certain minimum viscosity. Such oils, however, are unsuitable for use at very low temperatures on account of their greatly increased viscosity and inability to flow due to the separation of Waxy constituents at said very low temperatures. Thus typical internal combustion engine lubricants which are perfectly satisfactory in service under normal operating conditions may be quite unsuitable at temperatures below 0 F. as they do not possess a sufiiciently low pour-point.

If, on the other hand oils are selected which will flow freely at relatively low temperatures, it is found that they are quite unsuitable for use at high temperatures e. g, in internal combustion engines on account of their low viscosity, high volatility and lack of adequate lubricating properties.

It is an object of the present invention to provide lubricating compositions suitable for use over a wide range of temperatures and under a variety of conditions and suitable for use in internal combustion engines or other mechanisms where high temperatures are involved e. g.

in the lubrication of gas turbines, especially the propeller a turbine type where the lubricant is required to lubricate not only the bearings but also the reduction gearing, said lubricating compositions also being stable and fluid at low or very low temperatures e. g. down to 40 F. or even 80 F.

It has already been proposed to employ as lubricants (especially suited for use at low temperatures) certain diesters of aliphatic dicarboxylic acids with branchedchain aliphatic alcohols, a number of such esters being described in the journal entitled Industrial and Engineering Chemistry April 1947, p. 484 to p. 497.

These esters possess extremely low pour-points, have excellent viscosity-temperature characteristics and good lubricating properties and have been found very satisfactory lubricants for a variety of instruments such as watches, clocks, aeronautical compasses, gyroscopes, meters, gun-turret mechanisms and a variety of aircraft 2,836,565 Patented May 27, 1958 and other scientific instruments, especially where, as in aircraft, very low temperatures are encountered.

it has also been proposed to stabilise such esters by the addition of antioxidants such as organic amines and phenols of various types.

It has previously been proposed to stabilize these esters by the addition of minor amounts of metal salts of organic dithiophosphoric acids, with or Without the further addition of metal salts of oil-soluble petroleum sulphonic acids. By this means the esters could be stabilized against oxidation at relatively high temperatures, and rendered suitable for the lubrication of internal combustion engines and gas turbines.

Recent improvements in the design of gas turbine engines have increased the severity of the demands made upon the lubricant, which must have good oxidation resistance at high temperatures, e. g. of the order of 200 C., at the same time being non-corrosive towards metals used in the construction of the turbine engines, e. g. steel, copper, brass, magnesium, aluminium and cadmium-plated steel. It must also possess a high loadcarrying capacity, in order to provide adequate lubrication of the reduction gearing in the propeller-turbine type of engines.

While it is true that many additives which have been used in the past to improve the properties of hydrocarbons, e. g. mineral lubricating oils, in various respects are also efiective in dicarboxylic acid diesters for the same purpose, this is by no means universally true. Furthermore, the very high temperatures encountered in gas turbine engines, coupled with the presence of metals susceptible to chemical attack, greatly restrict the types of chemical additives which can be employed either as antioxidants or as extreme pressure agents. Thus, additives such as the metal salts of organic dithiophosphoric acids, and the dialkyl thioethers and dithiocarbamates which are among the most effective antioxidants for mineral lubricating oils used in internal combustion engines, are not entirely satisfactory when used in dicarboxylic acid diester type lubricants for gas turbine engines, due to lack of adequate thermal stability at the very high temperatures encountered.

The loss of these additives by thermal decomposition results in the rapid loss of oxidation resistance, While the liberated sulphur is capable of causing serious damage to the cuprous metals present. Other additives containing metals such as barium, though widely used in mineral lubricating oils for internal combustion engines, are not so suitable for use in gas turbine lubricants as they tend to promote the formation of carbonaceous deposits on very hot metal surfaces. This is particularly undesirable as, after switching off the engine, the oil soak temperature of the bearings may reach 350 C. or even higher, and it is important that the residual oil film shall volatilize without leaving an appreciable residue.

On the other hand, it may be possible to employ, in aliphatic dicarboxylic acid diesters, additives which by reason of lack of solubility, or for other reasons, have not found application in mineral lubricating oils. Examples of such additives are phenothiazine and some of its derivatives, as described in the journal entitled Industrial and Engineering hemistry, December 1950, p. 2479 to p. 2489. These compounds are undoubtedly very eliective antioxidants for diester type lubricants at high temperatures, but they suiier from certain serious disadvantages, notably a tendency to undergo photo-chemical oxidation with consequent precipitation of sludge, a tendency to promote lacquer and sludge formation at high temperatures, and a marked tendency to liberate sulphur or sulphur compounds corrosive towards cuprous metals both in the liquid and in the vapour phase.

It is an object of the present invention to provide antioxidants for dialkyl esters of aliphatic dicarboxylic acids which are effective at high temperatures, at the same time being relatively free from objectionable corrosive tendencies towards cuprous and other metals, from lacquer and deposit forming tendencies, and from sensitivity to light.

We have found that by the addition of minor amounts of hydroxy-substituted diaryl thioethers to the esters referred to above, their oxidation resistance at high temperatures can be greatly improved, and that by the further addition of a basic aliphatic, alicyclic or heterocyclic amine, having a boiling point of at least 140' C., a still further improvement can be obtained.

More specifically, this invention consists in a lubricating composition comprising a neutral ester or mixture of esters derived from aliphatic or cycloaliphatic dicarboxylic acids having from four to ten carbon atoms in the -molecule, and branched-chain aliphatic or alkyl-substituted cycloaliphatic alcohols having at least four carbon atoms in the molecule or from mixtures of such alcohols with glycols, and a minor proportion ofa hydroxy-substituted diaryl thioether.

The term neutral ester used throughout the specification and claims is tobe understood as referring to esters having an acidity not exceeding 1 mg. KOH per gm.

The preferred esters have the general formula COORi wherein R is'an aliphatic or cycloaliphatic hydrocarbon radical having from 2 to 8 carbon atoms, and R and R are the same or different and are branched-chain alkyl or alkyl-substituted cycloalkyl radicals having at least 4 carbon atoms. Examples of the radical R are CH: CH, (CI-i (where n is an integer from 2 to 8) and /CE2 -CH CH.CHs-

While esters having the above general formula are preferred, it is also Within the scope of the invention to employ esters obtained by esterifying the dicarboxylic acids with a mixture of an alcohol and a glycol to give complex esters having the general formula R OOC.R-COOR OC-RCOOR wherein R, R and R are as defined above and OH Ra is a glycol or polyglycol having not more than about 12 carbon atoms.

Other types of complex esters which may be employed are prepared by esterifying a dicarboxylic acid (1 mol) with a glycol (2 mols) and a monocarboxylic acid (2 mols) or with 1 mol each of a glycol, a monocarboxylic acid and a monohydric alcohol.

Esters may be employed derived from succinic, maleic,

pyrotartaric, glutaric, adipic, pimelic, suberic, azelaic, sebacic and pinic acids, specific esters being:

Di(l-methyl-4-ethyl octyl) glutarate Di(2-ethyl hexyl) adipate Di(3-methyl butyl) azelate Di(2-ethyl hexyl) azelate Di(2-ethyl hexyl) sebacate Di(3 :5 5-trimethylhexyl) sebacate Di(2-ethyl hexyl) maleate Di (methyl cyclohexyl) adipate 2-ethyl hexyl l-methyl heptyl sebacate Di-(Z-ethyl hexyl) pinate Whereas the majority of esters falling within the foregoing class possess both a high viscosity index and low pour-point, certain of them e g. di(1:3-dimethyl butyl) adipate, di(3-methyl butyl) sebacate have relatively high freezing points (above 0 F.), and therefore would not normally be suitable for the purposes of this invention, except in admixture with other esters of lower freezing point.

It will be understood that different esters may be selected according to the conditions under which the lubricant is to be used. Thus for use at very high temperatures it will be preferred to employ the esters of high molecular Weight and particularly the higher branchedchain di-esters of azelaic and sebacic acid.

The complex esters which may be employed are preferably prepared by esterifying 2 mols. of the dicarboxylic acid, 2 mols. of the branched-chain aliphatic or alkylsubstituted cycloaliphatic alcohol, and not more than 1 mol. of glycol. Examples of glycols which may be used are ethylene, propylene and hexylene glycols and the polyglycols, e. g., diethylene, triethylene, and tetraethylene glycol. I

When a monocarboxylic acid is used in preparing a complex ester this should be a straight or branched-chain aliphatic acid having from 2-10 carbon atoms inclusive.

Specific examples of' suitable complex esters which may be employed in accordance with the present invention are:

Ester prepared from ethylene glycol (1 mol), sebacic acid (2 mols) and 2-ethyl hexanol (2 mols) Ester prepared from triethylene glycol (1 mol), adipic acid (1 mol), n-caproic acid (1 mol) and 2-ethyl hexanol (lrnol).

While either the simple diesters of dicarboxylic acids or the complex esters may be employed in accordance with the present invention, these materials have certain limitations. Thus the simple esters have rather too low viscosities to be suitable as such for lubricating the existing types of aircraft gas turbines which normally require a lubricant having a minimum viscosity of 7.5 centistokes at 210 F., whereas the complex esters in general have insufliciently low pour-points which should desirably be of the order of F. or below.

Accordingly, in a preferred form of the present inven tion the dicarboxylic acid diesters of. the type described are thickened to the desired degree by the addition of polymers of esters of acrylic or alkyl-substituted acrylic acids, e. g. methacrylic acid. Compositions of this type can be produced having sufliciently high viscosities combined with very low pour-points.

The polymers are employed in proportions of from 1% to 15%, preferably from 5% to 10%, by weight on the weight of the final composition.

Examples of polymers which may be employed are the polymerised n-hexyl, 3:5:5-trimethyl hexyl and lauryl esters of methacrylic acid, and various commercially available materials sold under the trade name Acryloid. It is preferred to use polymers which are not only freely soluble in the diester lubricants but also miscible with mineral oil.

The hydroxy-substituted diaryl thioethers to be employed in accordance with the present invention have the general formula pp'-Dihydroxy diphenyl thioether Di 3 -methyl-4-hydroxyphenyl) thioether Di(4-methyl-3-tertiary butyl-Z-hydroxyphenyl) thioether Thioether prepared from tertiary butyl catechol Di 3-methoxy-4-hydroxyphenyl) thioether Thioether prepared from o-phenyl phenol These compounds can be prepared by well known methods such as by the action of sulphur dichloride on the desired phenol in an inert solvent such as benzene or carbon tetrachloride or, in the case of the last compound, by the action of sulphur dichloride on o-phenyl phenol in the presence of a metal catalyst such as copper powder or iron filings. Their use in mineral lubricating oils has been known for some considerable time, but although possessing antioxidant properties to some degree they have not found wide application in internal combustion engine oils and have been considered inferior to the metal dithiophosphates and other metal-containing additives. It is important that compounds for use in conjunction with the diesters of the present invention should be substantially free from the corresponding diand poly-sulphides and from free sulphur.

In view of the fact that these thioethers have had such limited usefulness as antioxidants in mineral oils, it is not a little surprising to find them so eifective as antioxidants in diester type lubricants. Moreover, whereas thioethers derived from alkylated phenols, e. g., tertiary butyl phenols and cresols (which themselves have antioxidant properties) have apparently always been preferred for use as antioxidants for mineral oils, we have found that for the purposes of the present invention the best resultsare obtained by the use of thioethers derived from phenols which are unsubstituted or substituted only by short alkyl radicals such as methyl groups and preferably by not more than one methyl group, although the use of more highly alkylated phenols is not excluded. This preferred class of phenol thioethers have good solubility in the esters of the present invention, but their solubility in most mineral oils is so limited that their use as antioxidants in that connection would hardly be considered.

In addition to the simple thioethers, examples of which have been quoted, it is possible to use polymeric compounds such as those obtained by the action of more than one (but less than 2) mols. of sulphur dichloride, on 2 mols. of a phenol having at least two free orthoor para-positions in the nucleus.

It will be understood that the compounds selected for use must have adequate solubility in the esters in which they are to be dissolved. Thioethers in which the radicals Ar in the above general formula consist .of condensed aryl nuclei, e. g., naphthyl groups, have inadequate solubility unless compensating nuclear alkyl substituents or ether groups are present. Similarly, compounds in which x, y or z are more than 1 tend to lack solubility and must be compensated in the same way. Generally it is preferred to employ compounds in which the radicals Ar are phenyl radicals and x and z are 1, n being zero.

While blends containing the hydroxy-substituted diaryl thioethers described exhibit good stability towards oxidation at high temperatures, there may be some tendency for the additives to break down with consequent 5 loss of sulphur which is potentially corrosive, especially to cuprous metals. As already indicated we have found that by the inclusion of certain amines the corrosion of cuprous metals can be reduced, increased stability towards oxidation being at the same time imparted to the blend. Although amines boiling below' C. may also be effective in this respect, they would rapidly be lost from the fluid at high temperatures, and therefore it has been found desirable to employ amines having a boiling point of at least 140 C. and preferably exceeding 200 C. Examples of suitable amines which may be used are n-octylamine and higher primary aliphatic amines, dibutylamine, and'higher secondary aliphatic amines, cycloaliphatic amines such as dicyclohexylamine, and heterocyclic amines such as the lupetidines and higher homologues of piperidine. Tertiary amines having basic properties may also be employed, e. g., di-n-octylmethylamine and 2:6-lutidine.

Generally these amines will be employed in relatively small amounts, e. g., from 0.01 to 1.0 percent by weight on the weight of the final composition, and preferably from 0.05 to 0.5 percent by weight on the weight of the final composition.

The preferred lubricants of this invention, which are designed for the satisfactory lubrication of jet engines of the propeller-turbine type, include a hydroxy-substituted diaryl thioether and an amine of the type described.

Other additives may be included if desired, e. g., rust inhibitors such as very small amounts of metal petroleum sulphonates, or other known compounds capable of inhibiting rust formation in presence of water and extreme pressure agents.

It is also Within the scope of the invention to include lubricants in which a certain amount of mineral oil is added to the composition.

The amount added and type of mineral oil will depend upon the pour-point required for the particular application in mind, but in any case the amount of mineral oil present should not exceed 50% by weight on the weight of the composition.

Where extremely low pour-points of the order of 80 F. are required, the presence of mineral oil may be inadmissible.

Specific compositions which may be employed in accordance with the present invention are exemplified in the tables of test results which follow.

To determine the oxidation stability of the compositions of this invention at elevated temperatures, a modification of the well-known British Air Ministry oxidation test was employed. This test was carried out precisely as described in tandard Methods for Testing Petroleum and its Products, Institute of Petroleum, 1949, Method I. P. 48/44, with the exception that various temperatures were employed C, C., or 200 C.) and that there was present a copper catalyst consisting of a piece of polished copper foil 2 inches x 1% inches rolled into a cylinder as described'in I. P. Method 56/46, this being replaced by a fresh catalyst'half way through the test, i. c. after 6 hours in the case of tests at l60 C. and 180 C. and after every 3 hours in the case of tests at 200 C. Additionally, in tests at 200 C. only, there were present strips of polished copper foil 6 inches x /2 inch suspended in the vapour phase above the oil, the lower portion of the strips being folded into three segments approximately 1 inch long, so that the overall length of the strip above the oil was about 3 inches and the lower end of the strip was 3 /2 inches above the surface of the oil.

After air blowing, the percentage increase in viscosity and acidity rise were determined. Also, in the case of tests at 200 C., the vapour-phase copper strips were examined for signs of attack by volatile acids, as evidenced by the presence of a green colour.

The insuing tables illustrate the efiectiveness of the compositions of the present invention.

Table 1.--Tests at 160 C.

Viscosity at 100 F. (cs.) Percent, Acidity Test N o. Ester or Ester Additives Viscosity (mgs.

blend Increase KOHI Before After gram) Test Test 1 D1 (2. ethyl None 12.56 18. 89 50.4 9.0 hexyl) sebaeate.

2 r1n 0.05% pp '-d1hydroxy- 12.54 12.73 1.5 0. 22

diphenyl thloether. 3 D1(2.ethy1hexy1) None 7. 37 14. 07 90.9 5.6

' adipate. 4 0.5% thioether prepared 7. 17 7. 86 9. 6 0. 28

from o-phenyl phenol.

N. B. The acidities of the samples of esters employed, before oxidation, were: 20 Mgs. KOI-I per gram Di(2-ethyl hexyl) sebacate 0.22 Di(2-ethyl hexyl) adipate 0.22

Table lI.--Te'sts at 180 C.

Viscosity at 100 F. (cs.) Percent Acidity Test No. Ester or Ester Blend Additives Viscosity (mgs.

Change KOHper Before After gram) Test Test 5 Di(3:5:5-trimethyl hexyl) None 18. 16 26. 41 +45. 4 .10. 0

sebacate.

0.5% 2:6 di-t-butyl-et- 19.16- 23.12 +202 7.8

methyl phenol. 0.25% pp-dihydroxy- 19.33 20.72 +7.2 0.45

diphenyl thioether. 0.25% pp'-dihydroxy- 19.35 19.71 +1.2 0.17

diphenyl thioether +01% dicyclohexylamme. 90% D1(2'ethy1 hexyl) 9 maleate. None 31.49 53. 83 +7L0 4.5

10% Aeryloid 710 10 --d0 1% thioether prepared 29.53 85.08 +18.8 1.95

7 D1( 11 1 1 from t-butyl cateehol. 50 0 met y eye 0.57 p '-dihydroxyd1- 11 3 8 gih enylt oether +01% 10.53 20.17 +3.4 0.50

hexyl Sebacate -n-octyl methylamme. 44% Di(2-ethy1 hexyl) so f l i 's 5 t 1m th 1 1 r e y 12 Sebacate None 35. 36 33. 84 4.3 7.3

6915 tPolynorlyl methacrya e. 13 d0 1% Polymeric thioether 36.77 38.10 +3.4 L68 prepared from p-oetyl phenol. 14 240th? hexyl capryl suc- None 7.26 9. 54 +31.6 8.7

01110. e. 15 do 0.5%,1 1:3 xylen-A-ol-thio- 7.38 7. 92 +7.3 2.0

e er. 97% D1(2-ethy1 butyl) 16 azelate. None 11.83 13.76 +1.6.3 6.2

8% Aarylold 710 17 d0 0.5% p 'dihydroxydl- 12.04 12. 47 +3.6 1.12

phenyl 1: leather. 47%b Dti(2-ethyl hexyl) se aca e 2.07 p 'dlhydroxydi- 1s 47% Dims-5 methyl fien l t oether. 40.11 40.44 +0.7 0.45

mm Sebacate- 0 01710 etldine 67ftPo1yn0ny1 methacry- D N. B. The acidities of the samples of esters employed, before oxidation, were:

Mgs. KOH per gm.

Di(2-ethy1 hexyl) maleate 0.28 Di(methyl cyciohexyl) adipate 0.14 2-ethyl hexyl capryl succinate 0.11

Di(2-ethyl butyl) azelate 0.11

The polymeric thioether used in test No. 13 was prepared from 1.2 mols. p-octyl (uwy' -tetramethyl butyl) phenol and 1 mol. sulphur dichloride. I 7

The Z-ethyl hexyl capryl siiecinate used in tests 14 'and 15 was a mixed ester prepared from succinic acid (1mo1.), capry1 alcohol (l-methyl heptanol) (111191.), and 2ethy1,hexa11ol (1 mol). I.

a 10 I) Acryloid 710 was a commercially available mineral The initial acidity of the diesters Was an important oil concentrate of a mixture of polymethacrylate esters. factor in their oxidation stability, and it was generally The polynonyl methacrylaic used Was a comparatively desirable to employ esters having an initial acidity not low molecular Weight polymer of 3:5:5-t1imethyl hexyl exceeding 0.3 mg. KOl-l per gram.

methacrylate.

Table III.Tests at 200 C.

Attack of Percent Acidity vapour Test N o. Ester or Ester Blend Additives Viscosity (mgs. phase cop- Change KOH per per strips gram) observed after-- 4gggig-ethytl hexyllsellilacatg 5 i :5:5 rimet y exy T 19 sgbacatei None 10.0 .5 3 hou.s.

6% Polynonvl rnethacrylate do 1 1 1% pg-dihydroxy-diphenyl thi- +10. 4 0. 78 Absent.

e er. 1% 1:3-Xylen-2-ol thioether +8. 4. 1 D0. 7 D 1% 1:3-xylen-4o1 thioether +15.-4 10. 6 9 hours.

0 477 Di(3:5:5trimethyl hexyl) 0.57 pp dihydroxy-diphenyl s bacate. th ioether. 5 68 Absent" 6% Polynonyl methacrylate..- 24 do 1% Thioether prepared from +6.9 1.90 Do.

guaiacol. 25 do 1% Di(3-methyl4-hydroxyphen- +8.6 1. 78. D0.

yl) thioether +01% Dicyclohexylamine +0.05% Calcium petroleum sulphonate. 1% Di(5-methyl-2-hydroxyphen- 26 do yl) thioether. +4.4 1.12 Do.

7 0.1% Dicyclohexylamine 90% Di(2-ethyl hexy1)sebacate 27 4.5% Polylauryl methacrylate None -23.5 4.5 3 hours.

5.5% Mineral 011A 28 do 2% 1:3xyle11-2-ol thioether +108 5.0 Qhours. 29 do 2% 1:3 xylen-2-ol thioether+l% +1. 2 l. 68 Do,

Oleylamine.

Mineral oil A was a solvent refined mineral oil of viscosity about seconds Redwood at 140 F.

The oleylamine used in test 29 was a commercially available material containing approximately unsaturated G15 amines, 10% saturated C amines, and 10% lower amines (C and 01 about 88% of the amines being primary.

Table I V.A ll tests were carried out for 22 hours at Cadmium-plated steel test-piece with copper Copper test-piece with steel catalyst catalyst Test Ester or Ester Blend Additives Colour of- Acidity N0. Wt. after Vt.

change (mgs. change Colour of Acidity 5 KOH/ (mgs.) specimen afterspeeimen Catalyst gram) 3Q Di(3:5:5 trimethyl hexyl) None -252.5 Corroded Light Pea- 10.1 +1.2 Peacock... 9.6

sebacate. cock. 31 do 0.5% Dl(3-methyl-4-hy- +0.6 Light yellow" Light 0, 50 +3.1 Brown 0. 50

droxyphenyl) thioether. brown. 0. 5% Di(3-methyl-4-hy- Brown, 32 do droxyphenyl) thioether. Nil. do cock p o, 17 +3,o do on 0.2% Tri-n-decylaminet? i fftt 'tt oxyp eny 10c er. 33 do 7? Di n octylmethyla -22 mm 0.5% Di(3-methyl-4-hy- 34 do droxyphenyl) thioether. +0.5 .do Brown 0.11 +2.8 -do 0.11

0.1% Di-n-octylaminc 0.5% Dl(3-methyl-4-l1y- 35 -do droxyphenyl) thioether. +0.5 -do Peacock"-.. o, 11 +14 tl0- 0.11

0. 1% 2:6 Lutidine 47% Di(2-ethyl hexyl) ir y i i s 5 t in: 111 1 0057 o 11 t 1 i2: rey acumperoeum 3s Sebacam sulphonate. Light reY Bright 16.0 1.4 Peacock..- 11.5

6% tPolynonyl methacry- 0.051% Ctalcium petrosulp ona e. Dun Dun 37 --do l.07 pp-D1hydroxyd1- +0.2 Grey 0.28 +1.0 0.34

plienyl thioether. Copper Copper 0.2% Dicyclohexylamine-. 0.05;? Calcium petrosul- B1 k p ona e. as 38 "d0 5% P46111217 1 Clean Black 0- Patchy nol disulphide. 40% D1(3:5:5 trimethyl v hexyl) adipate. 1% D1(3-methyl-4- hydrox Dark 0 28 39 40% Di(Alpha11ol 79) adiyphenyl) thioether 2.6 Bufi Patches Brown 0.28 3.4 smock pate. +0.2%n-Dodecylainine. P 20% Mineral oil B 47% Di(2-etl1yl hexyl) seb- D k Patchy Bufiat l Dark pea- 40 47% "methyl 0.5% Phenothiazine -54.s corroded 0.22 +0.2 cock 0.17 heiryl) sebaeate. appearance black brown 6% Polynonyl methacryj patches.

late.

1 In test No. 36 a jelly-like deposit had formed on the catalyst and elsewhere in the beaker. In test No. 40 the oil became very dark.

In the foregoing tables, tests 1, 3, 5, 9, 12, 14, l6, l9

and 27 indicate the fairly rapid oxidation undergone by various uninhibited esters and blends with polymethacrylate type thickening agents, While test No. 6 illustrates the comparative ineflectiveness at high temperatures of an additive typical of those heretofore proposed. It was noteworthy that when severe oxidation took place blends containing polymers often showed a decrease in viscosity due to degradation of the polymeric thickening agent.(tests 12, 19 and 27).

The efiectiveness of a variety of hydroxy-substituted aromatic thioethers is amply demonstrated by the foregoing tests, and the additional benefit from the inclusion of an amine of the type described can be seen by comparing tests 7 and 8, and tests 28 and 29.

A general survey of the test results indicates the specially effective inhibition of oxidation achieved by the use of thioethers derived from phenol and the cresols, as in tests 2, 7, 8, 11, 17, 18, 20, 23, 25 and 26, other thioethers-even those derived from the xylenols' (tests 15, 21, 22 and 28) being distinctly less effective.

Some further tests were carried out using the procedure described in Ministry of Supply Material Specification No. D. Eng. R. D. 2487 (Issue No. 2, dated January 1,

1954), paragraph 7.4. This test was a combined oxidation and corrosion test designed to measure the tendency of an oil to develop corrosive oxidation products at.

' blackening of the copper specimen and catalyst due to the liberation of active sulphur, while test 40 demonstrated that phenothiazine, hitherto regarded as the most effective high-temperature antioxidant for synthetic diester-type lubricants (see Journal of Industrial and Engineering Chemistry, December 1950, pages 2479-2489) not only produced an undesirable amount of tarnishing of the copper specimen and catalyst, but attacked the cadmiumplated steel specimen very severely.

We claim: 7

1. A lubricating composition comprising a major proportion of at least one ester selected from the group consisting of di(2-ethyl hexyl) azelate, di(2-ethyl hexyl) sebacate, di(3:5:5-trimethylhexyl) azelate and di(3:5;5- trimethyl hexyl) sebacate and from 0.05% to 2.0% of pp'-dihydroxydiphenyl thioether.

2. A lubricating composition comprising a major proportion of at least one ester selected from the group consisting of di-(2-ethy1 hexyl) azelate, di(2-ethyl hexyl) Tests 30 and 36 are illustrative of the very high acid- 1 tion under the conditions. of this test, resulted in severe sebacate, di(3:5:5-trimethyl hexyl) azelate and di(3:5:5- trirnethyl hexyl) sebacate, from 5% to 10% of polynonyl methacrylate and from 0.05% to 2.0% of pp'-dihydroxy diphenyl thioether.

3. A lubricating composition comprising a major proportion of at least one ester selected from the group consisting of di(2-ethyl hexyl) azelate, di(2-ethyl hexyl) sebacate, di(3:5:5-trimethyl hexyl) azelate and di(3:5:5-

trimethyl hexyl) sebacate, from 5% to 10% of polynonyl methacrylate, from 0.05% to 2.0% of pp'-dihydroxy diphenyl thioether and from 0.05% to 0.5% of dicyclohexylamine.

References Cited in the file of this patent UNITED STATES PATENTS 7 2,139,766 Mikeska et a1 Dec. 13, 1938 2,209,463 Leiber et a1. July 30, 1940 2,229,858 McConnell Ian. 28, 1941 2,234,096 Teter Mar. 4, 1941 2,375,007 Larsen et a1. May 1, 1945 2,481,372 von Fuchs Sept. 6, 1949 2,559,521 Smith et al. July 3,1951 2,575,195 Smith et al. Nov. 13, 1951 OTHER REFERENCES Polymeric Additives for Synthetic Ester Lubricants, article by Glavis, I. & E. 'Chem., December 1950, pages 2441-2446. F 1 1 

1. A LUBRICATING COMPOSITION COMPRISING A MAJOR PROPORTION OF AT LEAST ONE ESTER SELECTED FROM THE GROUP CONSISTING OF DI(2-ETHYL HEXYL) AXELATE, DI(2-ETHYL HEXYL) SEBACATE, DI(3:5.5-TRIMETHYLHEXYL) AZELATE AND DI(3:5:5TRIMETHYL HEXYL) SEBACATE AND FROM 0.05% TO 2.0% OF PP - DIHYDROXYDIPHENYL THIOETHER. 